Oil globules are discrete sub-cellular organelles surrounded by a monolayer of amphipathic phospholipids, glycolipids or sterols that encircle a hydrophobic core of neutral lipids. They are ubiquitous in animals, micro-organisms and plants. In many micro-organisms, such as yeasts, microalgae and bacteria, the accumulation of oil globules appears to be induced specifically in response to environmental stresses such as nutrient limitation, high radiation or osmotic stress (Murphy D. (2001), Prog Lipid Res 40:325-438; Zhekisheva et al., (2005), J Phycol 41:819-826). Different models for oil globule biogenesis were postulated and are still in debate. Nevertheless it is commonly accepted that globules arise by vesiculation from the ER (Walther and Farese Jr., (2009), Biochim Biophys Acta Mol Cell Biol Lipids 1791:459-466).
Plant oil globules contain specific populations of proteins that are more or less tightly bound to their surface. Globule proteins described in the literature were suggested to play different roles, including globule formation, degradation, stabilization and globule-globule or globule-other organelles interaction. In Drosophila, yeast and mammalians, globules were also shown to compose refugee proteins which are not directly linked to lipid metabolism. Oleosin is the most abundant oil-body-associated protein family identified in plants but until today it has not been found in algae. Caleosin, a calcium binding lipid-body protein that was found to be associated also with ER membranes, is ubiquitous among higher plants.
Some unicellular algae are known to be able to deposit very large amounts of oil in oil globules. Oil accumulation is usually accompanied by cessation of cell growth and in some species oil can account for as high as 60% of the cell dry mass. To date little is known about algal oil globules and especially about their protein composition and their potential roles.
The unicellular green alga Haematococcus pluvialis is well known as the best natural source for the high value red pigment astaxanthin. This carotenoid is accumulated in cytoplasmic oil globules, under inductive conditions. Accumulation of astaxanthin in H. pluvialis is positively correlated with lipid accumulation; the former depends on the latter but not vise-versa. Lipids accumulation also depends on de novo fatty acid synthesis. Under nitrate deprivation astaxanthin and fatty acid content can reach up to 4% and 40% of cell dry mass, respectively. The build up of oil globules in H. pluvialis was also found to be structurally related to ER membranes.